The invention relates to a method for diagnosing the dynamics of an exhaust gas sensor embodied as a lambda probe, which is disposed in an exhaust gas duct of an internal combustion engine in the direction of flow of the exhaust gas upstream or downstream of a catalytic converter and with which the air/fuel ratio of the gas mixture supplied to the internal combustion engine is controlled via a control circuit.
The invention further relates to a device for carrying out the method according to the invention.
A lambda control in combination with a catalytic converter is today the most efficient method for emission control of an Otto engine. Very low emission levels can be achieved only through their interaction with currently available ignition and fuel injection systems.
The use of a three-way or selective catalytic converter is particularly efficient. This catalytic converter has the capability of breaking down hydrocarbons, carbon monoxide and nitrogen oxides up to more than 98% if the engine is operated in a range of 1% around the stoichiometric air/fuel ratio where λ=1. The lambda value thereby indicates how far the actually present air/fuel mixture deviates from the value λ=1, which corresponds to a complete combustion of a theoretically required mass ratio of 14.7 kg air to 1 kg gasoline, i.e. the lambda value is calculated by dividing the air mass supplied by the theoretically required amount of air. In the case of excess air, λ>1 (lean mixture). In the case of excess fuel, λ<1 (rich mixture).
Lambda probes are used in modern internal combustion engines, which determine the oxygen concentration in the exhaust gas of the internal combustion engine and control the air and fuel supply to said internal combustion engine such that a composition of the exhaust gas is achieved, which is optimal for the exhaust gas aftertreatment by the catalytic converters provided in the exhaust gas duct of said internal combustion engine. In so doing, the lambda value is preferably controlled to 1, i.e. to a stoichiometric ratio of fuel to air. The pollutant emissions of said internal combustion engine can thus be minimized.
Different forms of lambda probes are in use. In the case of a two point lambda probe, also known as a bistable sensor or a Nernst sensor, the characteristic curve at λ=1 has a steplike descent. For that reason, said two point lambda probe can essentially discriminate only between rich exhaust gas during operation of the internal combustion engine with excess fuel and lean exhaust gas during operation with excess air.
A wideband lambda probe, also known as a continuous-action or linear lambda probe, allows for the measurement of the lambda value in the exhaust gas in a wide range around λ=1. An internal combustion engine can also, for example, thereby be controlled to a lean operation with excess air.
A rapid control of the exhaust gas composition to a predetermined lambda value is essential for the low-emission operation of the internal combustion engine. This is also especially true for internal combustion engines with cylinder-specific control, wherein the air/fuel mixture is individually adjusted for each cylinder of said internal combustion engine on the basis of the signal of the common lambda probe. In so doing, the lambda measurement must take place with a high temporal resolution in order for the compositions of the successive exhaust gas volumes of the different cylinders arriving at the lambda probe to be determined and to be associated with a respective cylinder.
Beside the selected control parameters of the lambda control circuit and the distance parameters, the dynamics of the lambda probe determine the speed of the control circuit. At the same time, the dynamics of the lambda probes, when they are new, are also sufficient for a cylinder-specific control with a common lambda probe for all cylinders in a common exhaust gas duct. Due to the effects of ageing, the dynamic characteristics of the lambda probes can, however, change to such an extent that the reaction speed of said lambda probe when determining the exhaust gas composition is no longer sufficient to ensure low emissions. If the emissions then lie outside the legal requirements, it is within the scope of the on-board diagnostics of the internal combustion engine to detect the faulty dynamics and provide for a corresponding error message.
In the German patent publication DE 102 60 721 A1, a method for diagnosing the dynamic properties of a lambda probe which is used at least intermittently for cylinder-specific lambda control as well as an associated diagnostic device is described. The method is thereby characterized in that at least one manipulated variable of the lambda control is measured and compared with a predefinable maximum threshold and, in the event of an exceedance of the maximum threshold, the dynamic behavior of the lambda probe is evaluated as being insufficient with regard to usability for the cylinder-specific lambda control. The dynamic properties of the lambda probe can even be detected from the cylinder-specific control because the cylinder-specific controllers diverge when the dynamics of the lambda probe are insufficient. Furthermore, a test mode of operation with a targeted disturbance or detuning of the current lambda value can be provided. Said method is, however, only suitable for internal combustion engines with cylinder-specific lambda control or else a targeted manipulation of the lambda value is required.
Other diagnostic methods are known for determining the dynamic properties of lambda probes. Hence, a measured lambda signal can, for example, be compared with an expected lambda signal in the case of a known excitation.
A disadvantage with many known methods is that only a change in the time constant of the lambda probe can thereby be detected but not a pure dead time in the probe signal. When the lambda probe is periodically excited, it is for instance not possible to detect a pure dead time in a comparison between the measured and the expected lambda signal. This is the case because there is no possibility to decide whether an observed reaction in the measured lambda signal can be traced back to the excitation of the directly preceding period or to an earlier period.
When diagnosing two point lambda probes, the evaluation of the duration of the resulting controller oscillation is known. Due to the steplike change in the output signal of the two point lambda probe at λ=1, a linear control, as it is used for wideband lambda probes, is not possible. The lambda control circuit for two point lambda probes is therefore generally carried out by a two point control algorithm. In this control algorithm, the signal passage through a threshold in the rich direction causes a step change in the manipulated controller variable toward lean, whereupon the fuel/air mixture and consequently the exhaust gas composition is changed with a constant gradient in the direction of lean. If the lambda signal passes through a second threshold in the lean direction on account of this change in the actuating variable, the manipulated controller variable then jumps again toward rich, followed by a change in the fuel/air mixture and consequently in the exhaust gas composition with a constant gradient in the direction of rich. In this way, a cyclical course of lambda signal and manipulated controller variable results, wherein the cycle duration depends on the distance parameters, the controller parameters and on the dynamics of the two point lambda probe. If the response characteristics of the two point lambda probe protract, for example due to the effects of ageing, this then leads to an extension of the cycle duration. If this exceeds a predetermined limit value, it can be concluded that the dynamics of the two point lambda probe are no longer sufficient.
With methods of this kind, symmetrical errors in the probe dynamics, i.e. the signal of the probe is equally delayed on both flanks, are in fact easily detected. The detection of asymmetric errors on the other hand is only possible to a limited extent with these methods. Asymmetric errors in the probe dynamics, of course, also lead to an extension of the cycle duration, but to such a limited extent that a reliable error detection using conventional methods is only possible in the case of very large errors. The asymmetric error in probe dynamics has, however, a greater impact on the exhaust gas than does a symmetrical error. For that reason, the detection of an asymmetric error represents a large challenge.
It is therefore the aim of the invention to provide a method, which allows for a reliable and improved on-board diagnostics of the dynamics of an exhaust gas sensor.
It is furthermore the aim of the invention to provide a corresponding device.